The present invention relates to systems for transmitting data input in digital form representing any kind of parameter, which can constitute, in particular, sound and/or images, the data being transmitted to and from mobile stations. The present invention also relates to the transmitters and receivers that form parts of such a system.
The term xe2x80x9cto and from mobile stationsxe2x80x9d should be interpreted as meaning that the system is suitable for accommodating the constraints that are specific to that circumstance, while nevertheless remaining entirely suitable for transmission when both the transmitter and the receiver are stationary.
The transmission and broadcasting of data by radio suffers from disturbances having a very wide range of origins and characteristics. Received signal attenuation increases with distance and with the presence of obstacles and can reduce power on reception to a value that is comparable to that of disturbances. Multiple echoes give rise to interference between successive symbols. They can be very numerous in an urban environment and they can lead to very high transmission delays in an open environment. Certain kinds of industrial interference are continuous, but they are generally concentrated in frequency. Other disturbances of very short duration can, on the contrary, occupy a wide frequency spectrum. Finally, in a context of transmission between a mobile transmitter and/or receiver, the transmission channel can vary in non-negligible manner over time (in terms of echo level, signal attenuation, etc.), and the speed of such variation increases with increasing travel speed.
Even before considering transmission or broadcasting to or from mobile stations, methods and systems were designed to increase immunity to interference. U.S. Pat. No. 3,988,677 proposes implementing concatenated Reed Solomon encoding associated with convolutional encoding and time interleaving. And then, for the purpose of broadcasting sound to mobile stations, the article entitled xe2x80x9cInterleaving or spectrum spreading in digital radio intended for vehiclesxe2x80x9d by D. Pommier et al., published in EBU Review, Technical No. 217, June 1986, proposes associating frequency interleaving with time interleaving while using a form of multicarrier modulation known as coded orthogonal frequency division multiplex (COFDM). That approach has been accepted in a standard known as DAB for broadcasting sound to mobile stations. By using a narrow passband (1.5 MHz) and modulation with a small number of states (MDP4), it is restricted to a low data rate. The DVB-T broadcast standard likewise makes use of COFDM principles, but with a wider passband (8 MHz) and modulation having a larger number of states (64 QAM), thus making higher data rates possible. However, that standard relates clearly to use with transmitters and receivers that are stationary since it does not define time interleaving and it uses a number of carriers that is large compared with the passband (a minimum of 1704 carriers for an 8 MHz passband).
The present invention consists in a system for transmitting digital data that is suitable for adapting to a very wide range of propagation conditions because of its ability to parameterize its characteristics. A major, although not exclusive, application of the invention lies in transmitting video data from a commentary vehicle or telemetry from a test vehicle.
For a commentary vehicle, the environment can change from one venue to the next, in particular the number and delay of multiple paths can change considerably. The transmitter can also be located at a stationary station or it can be carried by vehicles having a wide variety of speeds, giving rise to a correspondingly wide variety of Doppler spreads and effects. The present invention enables the transmitter and the receiver to be configured in such a manner as to make use on each occasion of a transmission signal that is adapted to various different types of transmission conditions.
The present invention also makes it possible to transmit data at a high rate (well above that defined by the DAB standard) in a context where the transmitter and/or the receiver is/are mobile. This ability to operate at a high rate in a mobile environment is provided firstly by time interleaving and secondly by the option to use a number of carriers that is small relative to the passband. Time interleaving serves to combat various disturbances that the transmission can encounter from time to time (due to a temporary change of environment created by the transmitter and/or the receiver moving). The option of using a small number of carriers, which implies a high rate of modulation on each of them, makes it possible to combat fast variations of the channel over time (Doppler spreading).
For these purposes, the invention provides a transmitter for a radio transmission system, the transmitter comprising:
an encoder receiving input data and supplying a digital stream whose content is the result of error correcting encoding;
a spreader module for spreading bits over a selectable number of carriers belonging to a group of carriers that are uniformly distributed over a channel, so as to constitute a frequency division multiplex;
a time interlacing module for providing time interlacing consistent with a spread given by the spreader module;
a frequency interlacing module operating on the carriers defined by the spreader module;
a phase modulator or possibly a phase and amplitude modulator;
a frequency-time transform module;
a guard time insertion module for optionally inserting guard times that increase the time periods of the symbols; and
a selector module or interface enabling a set of operating parameters to be selected that corresponds best to the propagation conditions and to the type of interference expected.
The term xe2x80x9cmodulexe2x80x9d should be understood as designating a hardware or software component enabling the dedicated function to be performed; in practice many of the modules can be combined in the form of an application specific integrated circuit (ASIC).
In an aspect of the invention, each set of parameters is defined by a particular combination of parameters constituted by encoding efficiency, time interleaving depth, number of carriers, type of modulation, and length of any guard times. In an embodiment, each set of parameters makes use of different values for all of the parameters. In another embodiment, some of the parameters are fixed or adjustable independently of the others, the selectable sets relating to pairs or triplets of parameters.
Before any particular mission, it is thus possible for the operator to select a set of parameters that is suitable for the nature of the transmission channel that is known a priori. For example, when numerous paths are present, it is preferable to adopt a large number of carriers to reduce the data rate on each carrier and to lengthen the guard times. In contrast, this selection should be avoided when the transmitter is carried by a fast-moving vehicle such as a helicopter, given the possibility of Doppler spreading occurring with its unfavorable effect on symbols that are too long. It is then preferable to use a small number of carriers, corresponding to a short symbol time, which will thus be less sensitive to fast time variations in the transmission channel.
In another aspect of the invention, the number of carriers transmitted in parallel and the bandwidth used are such that high transmission data rates (higher than that defined by the DAB standard) are possible with a mobile transmitter and/or receiver, insofar as the data rate on each transmitted carrier is high enough to combat Doppler spreading effectively.
The transmitter advantageously also has a module for inserting analysis symbols, examples of which are given below, which symbols are intended for use by the receiver.
The resulting digital stream is applied to a digital-to-analog converter followed by means for transposition to the transmission frequency.
In a system, the receiver performs operations that are the duals of those performed by the transmitter and likewise has an interface enabling parameter values to be selected in such a manner as to make them correspond to the parameters of the transmitter, with these values being transmitted over an instruction network or during a preliminary transmission that is performed using parameters fixed on default values.
A module for inserting analysis symbols on transmission makes it easier for the receiver to be synchronized in frequency and in time, and also makes it possible to analyze any interference that may be present in the frequency band used by the transmission system. In particular, it is possible to use a null analysis symbol, i.e. without any carrier being transmitted, thereby enabling the receiver to listen to interference in the absence of transmission, and consequently to determine which carriers are the most disturbed. In the frequent case where decoding is performed by an algorithm that seeks a likelihood maximum (Viterbi algorithm), determining which carriers are subject to most interference makes it possible to give them lower weighting coefficients in the decoding trellis, and thus attenuate the effect of the interference.
Furthermore, taking analysis symbols into account on reception can make it possible to determine an optimum selection of parameters on transmission. In some cases, these parameters can be returned to the transmitter via a return channel, for the purpose of obtaining a new adjustment.
In practice, a xe2x80x9cnullxe2x80x9d symbol is generally inserted at the beginning of each transmitted data frame, together with a symbol dedicated to synchronizing the receiver.
A system comprising a transmitter and a receiver of the kinds described above can provide a data rate that is greater than the maximum rate authorized by the DAB standard while still remaining adapted to conditions of use whether stationary or mobile, depending on the number of carriers used for transmission.